The important part is to align your XNA camera to the physical camera. For that we need the Attitude property from the motion API. In order to use it we must first apply some transformations. The last line is needed for landscape apps. Remove it If your app works in portrait mode.

What you have now is a View matrix. You can use it to draw your stuff with Model.Draw(...) or assign it to Effect.View. Sometimes this isn't enough. What I wanted was the actual orientation of the camera/phone as a 3D vector.My first thought was to use Vector3.Transform(...) and transform a Vector3.Forward using the orientation matrix. That didn't work. Then I tried to get the orientation.Forward & orientation.Up but that didn't work either. Finally i wrote a small method that extracts the two vectors from a view matrix.As you can see, you can use the results in order to create your own view matrix or use it in your Camera class.